English: A vacuum tube electrode used in electrotherapy, an obsolete Victorian-era medical therapy, used from about 1890s to the 1930s. The electrode is used to apply high voltage radio frequency alternating current (AC) electricity from a Tesla or Oudin coil to the body to treat various ailments. It consists of a partially evacuated glass bulb with a metal plate sealed inside with an insulated handle. A wire from the high voltage terminal of the coil is attached to the terminal (upper right) of the electrode, and the glass bulb is applied to the skin. The high potential ionizes the gas in the bulb, giving off a violet glow. Only a very small amount of current could pass through the capacitance of the glass wall of the tube into the patient, making the treatment safe. The text says if applied to the skin the patient feels only a warm sensation, while if applied through layers of clothing he feels a prickling from sparks. The patient held a ground electrode attached to the bottom of the coil to serve as a return path for the current. Several hundred thousand to one million volts and frequencies of 200 kHz to 2 MHz were used.